1. Field of the Invention
The present invention relates to standardized mechanical interface (SMIF) systems for reducing particle contamination, and more particularly to transportable containers which are sealable to prevent influence of external factors on the contents of the containers, and which have a clean interior region.
2. Description of the Related Art
A standardized mechanical interface (SMIF) system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto wafers and/or reticles. This purpose is accomplished, in part, by mechanically ensuring that during transportation and storage the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
The SMIF concept is based on the use of a small volume of controlled (with respect to motion, gas flow direction and external contaminants), particle-free gas to provide a clean environment for articles.
Further details of one proposed system are described in the paper entitled "SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING," by Mihir Parikh and Ulrich Kaempf, Solid State Technology, Jul. 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.1 .mu.m to above 200 .mu.m. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one micron (1 .mu.m) and under. Unwanted contamination particles which have geometries measuring greater than 0.05 .mu.m substantially interfere with 1 .mu.m geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.5 m and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles become of interest.
A SMIF system has three main components: (1) sealed pods, having a minimal volume, used for storing and transporting cassettes holding reticles or wafers; (2) enclosures placed over cassette ports and wafer processing areas of processing equipment so that the environments inside the pods and enclosures (after having clean air sources) become miniature clean spaces; and (3) a transfer mechanism to load/unload wafer cassettes from a sealed pod without contamination of the wafers in the wafer cassette from external environments.
Wafers are stored and transported in pods, and are transferred from a pod to processing equipment in the following manner. First, a pod is placed at the interface port on top of the enclosure. Each pod includes a box and a box door designed to mate with doors on the interface ports of the processing equipment enclosures. Then, latches release the box door and the enclosure port door simultaneously; the box door and the interface port door are opened simultaneously so that particles which may have been on the external door surfaces are trapped ("sandwiched") between the box and interface port doors. A mechanical elevator lowers the two doors, with the cassette riding on top, into the enclosure covered space. A manipulator picks up the cassette and places it onto the cassette port/elevator of the equipment. After processing, the reverse operation takes place.
The SMIF system has proven effective, both inside and outside a clean room, through use in semiconductor fabrication facilities and experiments. A SMIF system provides at least a ten-fold improvement over the conventional handling of open cassettes inside the clean room.
There are several areas in which conventional SMIF pods may be improved. First, conventional SMIF pods do not provide protection from electrostatic discharge (ESD). A contamination constraint is created by the need for conductivity; in molded plastic materials the preferred and most common ingredient for providing volumetric conductivity is carbon powder or fiber. Unfortunately, this material tends to slough off during sliding contact with another surface even if the surrounding plastic remains intact. Generation of particles in this manner is undesirable as they are likely to be transferred to many locations during normal cassette transfer and may ultimately migrate to wafer surfaces.
Second, the materials used to construct injection molded products and seals may release, through outgassing, materials which interfere with critical device fabrication processes. In this regard, studies with current generation 150 mm SMIF-Pods have shown significantly less surface contamination on wafers stored in pods than on wafers stored in ULPA filtered recirculating air-systems. Nevertheless, the materials approach used in pod construction can benefit from attention to internal outgassing sources. Third, particle generation: In the course of normal production processing, a cassette of wafers will be loaded/unloaded from a SMIF-pod several hundred times, and will be subjected to many transport and placement operations. Each of these operations, as well as pod opening/closing, can generate particles, particularly where contact occurs between the cassette and supporting/retaining surfaces on the pod door.
Fourth, sealing: the functional importance of sealing the SMIF-Pod door has historically been based upon the requirement of providing a still-air environment for the wafers, to prevent cumulative particulate and surface contamination conditions generated by the numerous air changes per minute and other transient conditions in the general clean room. In many tests conducted on SMIF-Pods it has been determined that a "proximity" seal is adequate for excluding particulate sizes above approximately 0.5 .mu.m, but a heavy concentration of smaller particles can in fact diffuse across small air gaps and eventually result in wafer surface contamination. Accordingly, a volume-compressible seal was incorporated in conventional pods to preclude diffusion-based particulate transfer. Subsequently, the development of SMIF-Based systems utilizing a controlled atmosphere condition for the pod and port-plate components has added additional requirements to sealing performance.
Conventional seals are formed using a poromeric, volume compressible material which is fundamentally a polyurethane foam of controlled porosity and durometer. This material was developed for use in applications such as disk drive gasketing, clean room filters and cabinet assemblies. Cycle testing has shown that this material had excellent properties in terms of compression set, resistance to abrasion and particulation, and dimensional accuracy. Nevertheless, trace gas analysis results now indicate that outgassing from this seal is excessive for certain (e.g., 64 Mb) type product applications.
Fifth, cleaning: Historically, cleaning of SMIF pods has been performed in a manner similar to that used for other semiconductor handling products using a combination of deionized wafer and a non-ionic surfactant followed by thorough rinsing and air drying. Based on SMIF-user experience to date, this has been an effective approach for particle build-up control down to at least the 0.3 .mu.m level.
Future trends, however, point to the need for a cleaning approach which could provide improved levels of cleanliness for smaller particles as well as minimizing any possibility of introducing contamination during cleaning, i.e. organics, particles and residual moisture. Several new technologies have recently emerged that could enable a "dry" cleaning process to be used for pod component cleaning. An improved cleaning system would also ideally be compact enough to be located in the Fab area itself. Appropriate design of the internal pod elements and the cleaning system would facilitate cleaning with a minimum of human interaction for disassembly/reassembly.